Fender Frontman 65R - power amp intermittent noise

[Overthruster]

Hi folks. I'm new to this forum but an old hand at audio electronics repair (30+ years) and I find myself really up against a puzzler with this one.

I have a Fender Frontman 65R guitar amp that produces sputter/static-like noise — intermittently (of course).
Here's some details:

• It's definitely the power amp: I've disconnected the preamp and shorted the input to the power amp and nothing changes.
• It's not thermal: Sometimes it starts immediately on power-up, sometimes it takes several minutes; circuit cooler has no effect, neither making noise better or worse, no matter what components I hit.
• At this point I've replaced every semiconductor device in the power amp (they were all very inexpensive and/or I already had them on hand) except the output transistors.

I have not tried opening the feedback loop to isolate where the noise is originating because I don't trust this amp's stability running at open-loop gain.  (If there's anyone who has a Frontman 65 and is willing to give it a go I'd be much obliged  )

I've done lots of web searching and haven't found anyone with this problem on this amp, so your input and experience would be much appreciated.

I'll attach a schematic for reference.

[g1]

At this point I've replaced every semiconductor device in the power amp (they were all very inexpensive and/or I already had them on hand) except the output transistors.

Hmm.  I was going to mention that many 'noisy power amp' issues in this model are solved by replacing diffy pair Q6 and Q7 but I guess you've already done that.

[Overthruster]

Oh yeah. The differential pair was the first thing I went for. That's a really common source of power amp noise issues.

[phatt]

Had something similar years back,, after much swearing turned out to be failing contacts in the main power switch. they were arching intermittently,,  Also had it happen to a Sansui hifi amp not so long back,  Rare but it can happen.
Phil.

[Overthruster]

Had something similar years back,, after much swearing turned out to be failing contacts in the main power switch. they were arching intermittently

Now that's an idea! I'll check that next. I've seen it happen with old Pioneer stereo receivers, though I haven't worked on one of those for ages.

[Overthruster]

Nice idea with the power switch but no go. I bypassed it and nothing's changed.

[Jazz P Bass]

Have you tried inserting a signal in to the Power Amp In jack?
By doing this you isolate the preamp from the power amp circuit.

[Overthruster]

Have you tried inserting a signal in to the Power Amp In jack?
By doing this you isolate the preamp from the power amp circuit.

Yep. Tried that and even with the input to the power amp shorted it keeps making noise.

[phatt]

Hum it might be a broken track.
What about,,
Wait till it's dark, (Really Dark) and just watch.

If it's a hairline crack with a high current it will flash when fault happens.
I have found that to save a whole lot of time wasting searching for that kind of fault.
I had one PCB that had cracked right under the edge of the solder pad and no way you could see it as the solder slightly overlapped the edge of copper trace. Worth a try.
Phil.

[Jazz P Bass]

"Yep. Tried that and even with the input to the power amp shorted it keeps making noise."
So it's in the power amp.
Or the power supply.
Have you monitored the power supplies when the issue occurs?

[Loudthud]

I'm not sure how much the Frontman 65 has in common with the Frontman 212R, but the 212R has problems with solder connections fracturing on the PCB. A close inspection under bright light and magnification might reveal the problem.

[Overthruster]

Thanks for all the suggestions, everyone. I'm pretty confident I can rule out the power supply — I've looked at the supply line voltages on a scope and there's no sign of any spikes or noise. I'll start investigating the possibility of bad traces and solder joints now. (I've already re-soldered dozens of joints "just in case" but I have many more that I haven't touched yet.)